1. Field of the Invention
The present invention relates to a method of crystallizing an amorphous silicon film, and more particularly, to a crystallization method using sequential lateral solidification (SLS).
2. Discussion of Related Art
Polycrystalline silicon (p-Si) and amorphous silicon (a-Si) are often used as active layer materials for thin film transistors (TFTs) in liquid crystal display (LCD) devices. Since amorphous silicon (a-Si) can be deposited at a low temperature to form a thin film on a glass substrate, amorphous silicon (a-Si) is commonly used in switching devices of liquid crystal displays (LCDs). Unfortunately, amorphous silicon (a-Si) TFTs have relatively slow display response times that limit their suitability for large area LCD.
In contrast, polycrystalline silicon TFTs provide much faster display response times. Thus, polycrystalline silicon (p-Si) is well suited for use in large LCD devices, such as laptop computers and wall-mounted television sets. Such applications often require TFTs having field effect mobility greater than 30 cm2/Vs together with low leakage current.
A polycrystalline silicon film is composed of crystal grains having grain boundaries. The larger the grains and the more regular the grains boundaries the better the field effect mobility. Thus, a silicon crystallization method that produces large grains, ideally a single crystal, would be useful.
One method of crystallizing amorphous silicon into polycrystalline silicon is sequential lateral solidification (SLS). SLS crystallization uses the fact that silicon grains tend to grow laterally from the interfaces between liquid and solid silicon such that the resulting grain boundaries are perpendicular to the interfaces. With SLS, amorphous silicon is crystallized using a laser beam having a magnitude and a relative motion that melts amorphous silicon such that the melted silicon forms laterally grown silicon grains upon re-crystallization.
FIG. 1A is a schematic configuration of a conventional sequential lateral solidification (SLS) apparatus, while FIG. 1B shows a plan view of a conventional mask 38 that is used in the apparatus of FIG. 1A. In FIG. 1A, the SLS apparatus 32 includes a laser generator 36, a mask 38, a condenser lens 40, and an objective lens 42. The laser generator 36 generates and emits a laser beam 34. The intensity of the laser beam 34 is adjusted by an attenuator (not shown) in the path of the laser beam 34. The laser beam 34 is then condensed by the condenser lens 40 and is then directed onto the mask 38.
Specifically referencing FIG. 1B, the mask 38 includes a plurality of slits “A” through which the laser beam 34 passes, and light absorptive areas “B” that absorb the laser beam 34. The width of each slit “A” effectively defines the grain size of the crystallized silicon produced by a first laser irradiation. Furthermore, the distance between each slit “A” defines the size of the lateral grains growth of amorphous silicon crystallized by the SLS method. Referring not to FIG. 1A, the objective lens 42 is arranged below the mask and reduces the shape of the laser beam that passes through the mask 38.
Still referring to FIG. 1A, an X-Y stage 46 is arranged adjacent to the objective lens 42. The X-Y stage 46, which is movable in two orthogonal axial directions, includes an x-axial direction drive unit for driving the x-axis stage and a y-axial direction drive unit for driving the y-axis stage. A substrate 44 is placed on the X-Y stage 46 so as to receive light from the objective lens 42. Although not shown in FIG. 1A, it should be understood that an amorphous silicon film is on the substrate 44, thereby defining a sample substrate.
To use the conventional SLS apparatus, the laser generator 36 and the mask 38 are typically fixed in a predetermined position while the X-Y stage 46 moves the amorphous silicon film on the sample substrate 44 in the x-axial and/or y-axial direction. Alternatively, the X-Y stage 46 may be fixed while the mask 38 moves to crystallize the amorphous silicon film on the sample substrate 44.
When performing SLS crystallization, a buffer layer is typically formed on the substrate 44. Then, an amorphous silicon film is deposited on the buffer layer. Thereafter, the amorphous silicon is crystallized as described above. The amorphous silicon film is usually deposited on the buffer layer using chemical vapor deposition (CVD). Unfortunately, that method produces amorphous silicon with a lot of hydrogen. To reduce the hydrogen content the amorphous silicon film is typically thermal-treated, which causes de-hydrogenation, which results in a smoother surface on the crystalline silicon film. If the de-hydrogenation is not performed, the surface of the crystalline silicon film is rough and the electrical characteristics of the crystalline silicon film are degraded.
FIG. 2 is a plan view showing a substrate 44 having a partially crystallized amorphous silicon film 52. When performing SLS crystallization, it is difficult to crystallize all of the amorphous silicon film 52 at once because the laser beam 34 has a limited beam width, and because the mask 38 also has a limited size. Therefore, with large size substrate, the mask 38 is typically arranged numerous times over the substrate, while crystallization is repeated for the various mask arrangements. In FIG. 2, an area “C” that corresponds to one mask position is defined as a block. Crystallization of the amorphous silicon within a block “C” is achieved by irradiating the laser beam several times.
Crystallization of the amorphous silicon film will be explained as follows. FIGS. 3A to 3E are plan views showing one block of an amorphous silicon film being crystallized using a conventional SLS method. In the illustrated crystallization, it should be understood that the mask 38 (see FIGS. 1A and 1B) has three slits.
As is well known, the length of the lateral growth of a grain is determined by the energy density of the laser beam, by the temperature of substrate, and by the thickness of amorphous silicon film (as well as other factors). The maximum lateral grain growth should be understood as being dependent on optimized conditions. In the SLS method shown in FIGS. 3A to 3E, the width of the slits is less than or equal to twice the maximum lateral grain growth.
FIG. 3A shows an initial step of crystallizing the amorphous silicon film using a first laser beam irradiation. As described with reference to FIG. 1A, the laser beam 34 passes through the mask 38 and irradiates one block of an amorphous silicon film 52 on the sample substrate 44. The laser beam 34 is divided into three line beams by the three slits “A.” The three line beams irradiate and melt regions “D”, “E” and “F” of the amorphous silicon film 52. The energy density of the line beams should be sufficient to induce complete melting of the amorphous silicon film, i.e., complete melting regime.
Still referring to FIG. 3A, after complete melting, the liquid phase silicon begins to crystallize at interfaces 56a and 56b between the solid phase amorphous silicon and the liquid phase silicon. Namely, lateral grain growth of grains 58a and 58b proceeds from the un-melted regions to the fully-melted regions, so the grain boundaries are substantially perpendicular to the interfaces 56a and 56b (for clarity, only region “D” is shown with interfaces 56a and 56b and grains 58a and 58b, however, it should be understood that regions “E” and “F” also have such features).
Lateral growth stops in accordance with the width of the melted silicon region when: (1) grains grown from interfaces collide near a middle section 50 of the melted silicon region; or (2) polycrystalline silicon particles are formed in the middle section 50 as the melted silicon region solidifies sufficiently to generate solidification nuclei. Further, the grown length of the laterally grown grains is usually 1.5 to 2 micrometers after the laser beam irradiation process. Namely, the grains laterally grown by the first laser beam irradiation can typically have the maximum length generally ranging from 1.5 to 2 micrometers (μm).
Since the width of the slits “A” (see FIG. 1B) is less than or equal to twice the maximum lateral growth length of the grains, the width of the melted silicon region “D,” “E,” or “F” is also less than or equal to twice the maximum lateral growth length of the grains. Therefore, lateral grain growth stops when the crystalline silicon collide in the middle section 50, as shown in FIG. 3B.
FIG. 3B is an enlarged view of a portion “C1” of FIG. 3A. As mentioned, the first grains 58a grow from left to right, i.e., from the interface 56a to the center 50, and the second grains 58b grow from right to left, i.e., from the interface 56b to the center 50. When these grains 58a and 58b collide with each other at the center 50, the lateral grain growth stops, thereby defining first grain regions “G1a” and “G1b,” and a first border “H1.” As discussed above, the grain boundaries in directionally solidified silicon tend to form perpendicular to the interfaces 56a and 56b of the solid phase amorphous silicon and the liquid phase silicon. As a result of the first laser beam irradiation, crystallized regions “D,” “E,” and “F” each having the first grain regions “G1a” and “G1b” and the first border “H1” are formed in one block. The number of these crystallized regions “D,” “E,” and “F” will vary depending on the number of the slits “A” of FIG. 1B.
FIG. 3C is an enlarged view of the portion “C1” and illustrates the mask adjustment for a next laser beam irradiation. In order to grow the grains, the mask 38 moves (relative to the amorphous silicon) to a position where the slit(s) “A” exposes a portion of the grain region “G1a,” the border “H1,” and the grain region “G1b.” Namely, when the width of the first grain regions “G1a” and “G1b” is 2 micrometers (μm), the slit “A” of the mask moves by 0.7 micrometers (μm) in a transverse direction (i.e., in the x-axial direction). Then, a second laser beam irradiation is conducted to grow grains formed by the first laser beam irradiation, thereby resulting in grains 58c as shown in FIG. 3D.
FIG. 3D illustrates a partially crystallized amorphous silicon film after the second laser beam irradiation. During the second laser beam irradiation, the second laser beam irradiates portions of the first grain regions “G1a” and “G1b” and previously unexposed amorphous silicon. The regions irradiated by the second laser beam are melted and crystallized, thereby defining second grain regions “G2a” and “G2b,” and a second border “H2” thereof. During that time, the grains 58a generated by the first laser beam irradiation serve as seeds for the second crystallization. Thus, the grains 58c are formed by lateral grain growth of the grains 58a in the second melted regions. Namely, the silicon grains 58c formed by the second laser beam irradiation continue to grow along the silicon grains 58a formed by the first laser beam irradiation. Meanwhile, new silicon grains 58d are grown from a newly formed interface 56c. The lateral growth of the grains 58c and 58d stop when they collide with each other at a line 50b of the second border “H2”.
Accordingly, by repeating the steps of melting and crystallizing, one block of the amorphous silicon film is fully crystallized to form grains 58e as shown in FIG. 3E. Furthermore, the amorphous silicon film is converted into crystalline silicon film by block-by-block crystallization. Namely, the above-mentioned crystallization processes conducted within one block are repeated block by block across the amorphous silicon film. Therefore, the large amorphous silicon film is converted into a crystalline silicon film.
While generally successful, the conventional SLS method described above has disadvantages. Although the conventional SLS method produces relatively large size grains, the X-Y stage or the mask must repeatedly move a few micrometers to induce lateral grain growth. Therefore, the time required to move the X-Y stage or the mask 38 occupies a major part of the overall process time. This significantly decreases manufacturing efficiency.
To reduce the process time, a mask having closely spaced slits is often used. When a mask has a shorter distance between adjacent slits the mask can have more slits per unit area. Therefore, such a mask can reduce the number of laser beam shots and can increase productivity. However, a mask having closely spaced slits limits the grain length that can be achieved. This can reduce the suitability of the resulting crystalline silicon film for the active layer of driving devices, such as data driving circuits and gate driving circuits. However, crystalline silicon film formed by masks having closely spaced slits can still be useful for active layers of LCD panel switching devices (such as TFTs). This is because short grains (i.e., less than 1 micrometer) are still useful in such applications.
Accordingly, when forming TFT active layers, masks having closely spaced slits are usually used to reduce the overall processing time. However, when forming active layers for driving devices a mask having wider spaced slits are usually required. Thus, two masks, one for the TFTs and one for the driving circuits, are beneficial in some applications.
The masks for driving circuits have a relatively small number of slits per unit area. Furthermore, to make large grains the laser beam patterns of the second laser beam irradiation are required to overlap the center of the grain regions “G1a” and “G1b” that were formed by the first laser beam irradiation (reference FIG. 3C). However, the masks for TFTs have a relatively large number of slits per unit area. Therefore, the process time of forming the TFT active layer is relatively small (the grain size is smaller than in the driving circuit).
In the conventional art, let us suppose that the mask moves a distance of 0.7 micrometers, i.e., the translation distance is 0.7 micrometers. Further, let us suppose that the length of the lateral grain growth is about 1.2 micrometers and that the slit width is 2 micrometers. Further assume that the distance between adjacent slits is 10 micrometers. Under these assumptions the laser beam will be shot about 16 times to crystallize one block. This process time is relatively long and the manufacturing efficiency is low.
Alternatively, if the distance between adjacent slits is 4 micrometers, the laser beam will be shot about 9 times to crystallize one block. Although the number of laser beam irradiation decreases, the resulting grains are smaller and the crystalline silicon film has more larger grain boundaries. These grain boundaries interrupt the carrier movement and decrease the carrier mobility. Therefore, such small grains are usually not adequate for driving circuits. Therefore, another mask for forming the crystalline silicon active layer adequate to the driving circuit is required. Using the different mask for the driving circuit, however, causes increasing the cost of production.
Therefore, a new method of crystallizing amorphous silicon using sequential lateral solidification (SLS) such that both TFTs and driving circuit active layer materials are formed at the same time and with improved manufacturing efficiency would be beneficial.